Abstract: Using the atmospheric boundary layer gradient observations, the second level radiosonde data, visibility observations, high temporal and spatial density of meteorological observation data along the highway, and NECP 1°×1°reanalysis data, the formation and maintenance mechanism for the persistent heavy fog during 31 November to 9 December 2013 are analyzed. The results show that the process is characterized by long duration, wide range, and low visibility with high concentrations of pollutants. Analysis of the synoptic weather pattern shows that Jiangsu province was under the control of continental high pressure, which sustained there for a long time. The fog occurred over the uniform and high pressure zone. Apparently such a synoptic pattern provides an important condition for the formation and continuance of the fog. Surface layer inversion, low-level downdraft and southeasterly flow of warm and wet air are also important reasons for the formation of this heavy fog. The double inversion layers with a large depth provide important thermal conditions favorable for the formation of heavy fog, while the intrusion of weak cold airmass directly triggered the outbreak and intensification of the fog.